A common method for separating out heavier components from a liquid mixture is the use of centrifugation, whereby the mixture is subjected to a centrifugal force field which causes the sedimentation of the components at different rates according to their densities and volumes. In a centrifuge for separating biological cells, for example, a suspension of cells in a liquid medium is placed in a centrifuge tube which is rotated rapidly about an axis some distance from the tube so as to cause sedimentation of the cells and other structures toward the bottom of the tube. Cells or other particles may thus be separated differentially by varying the time of centrifugation with the denser components sealing at the bottom of the tube first. Such differential centrifugation is commonly used, for example, to separate red blood cells from white blood cells owing to the greater density of the former. After disruption of cells into a heterogeneous liquid, centrifugation is also used to separate cellular constituent parts such as nuclei, mitochondria, ribosomes, etc.
Any heterogeneous liquid mixture may be subjected to centrifugation. A more clearly demarcated degree of separation may be achieved, however, by interposing a density barrier which only particles having a greater density may penetrate. For example, a blood cell suspension may be layered over a solution of albumin or sucrose whose density is between that of red and white cells, allowing the red cells but not the white cells to go through the density barrier which affords a better separation. Modifications of this technique, called density gradient centrifugation, include using a centrifuge medium having a number of layers of varying density or using a medium having a continuous density gradient along the length of the centrifuge tube. If the different densities of the medium encompass the range of densities represented by the different components desired to be separated, each component will come to rest in a layer of the medium whose density matches its own.
Centrifugation may also be used in a continuous flow process in which heavier or lighter components are removed from a feedstock. Centrifugal separators of this type are commonly used in the dairy and paper industries as well as in isotopic separation processes. Since the liquid which is centrifuged in these cases is the feedstock itself and not a special centrifugation medium, it is usually not possible to employ the density gradient technique as described above to enhance the degree of separation of the components. A cleaner separation may be achieved, however, by the use of multi-stage centrifugal separators in which the feedstock is continuously depleted of either the heavier or lighter components as it proceeds through the multiple stages.
In any of the centrifugal separation processes described above, it would be advantageous if the centrifuge could be operated so as to optimize the separation of a select component or components from the rest of the mixture. For example, in density gradient centrifugation, the centrifugation must proceed long enough for the different particles to localize in their individual density layers. How long this takes depends on the sedimentation constants of the particles, the rotational speed of the centrifuge, and the composition of the centrifuge medium. If the centrifugation is allowed to proceed for too long and at too high a speed, however, disruption of the desired layers as well as fragile components such as cells may occur. In accordance with the present invention, therefore, a quantity of detectable test particles having a sedimentation constant approximately equal to that of a select component of a mixture which it is desired to separate is added to the mixture before or during the centrifugation. Such test particles, depending upon the type of mixture component whose sedimentation rate they are designed to emulate, may be either molecules or larger particles to which is conjugated a tag enabling the test particle to be detected by electro-optical or other means. The position mid/or velocity of the test particles in the mixture as centrifugation proceeds is monitored by a scanning device which feeds the position data to a computer which then controls the speed and duration of the centrifuge so as to result in a desired localization of the component which it is desired to separate. By means of such adaptive control of the centrifuge, its operation may be optimized to separate out a select component even as the composition of the centrifuge medium varies. The technique is thus especially useful in those applications where centrifugation takes place in a native medium subject to a great deal of variability as opposed to an artificial centrifuge medium.
The present invention may also be used in continuous centrifugal separation operations to control the speed of each of the individual centrifugal separation stages so as to result in the optimum separation of a select component or components from the feedstock. Other variables which may also be controlled include impeller blade angles and rate of throughput into each stage. In a continuous separation operation, test particles are continuously added to the feedstock, and the concentration of the particles is continuously monitored in the product coming out of the separator. A plurality of different test particles may also be used with each having a sedimentation constant corresponding to a different component of the feedstock. Select components may then be optimally stripped from the feedstock by removing product frown select stages and operating the stages in accordance with the concentration of test particles in each product stream. In cases where it is desired to remove the test particles from the product, the particles may be composed or partly composed of a ligand having a binding affinity for specific molecules immobilized in a reaction column. The test particles may then be separated from the rest of the product by passing the product through the reaction column.
It is a primary object of the invention to employ a computer and electro-optical or radiation scanning devices in order to adaptively control the operation of a centrifugal separator.
It is a further object of the invention to operate a centrifuge in an adaptive manner which optimizes the separation of a select component from a heterogeneous mixture.
It is a further object of the invention to operate a centrifuge so as to localize a select component of a heterogeneous mixture in a centrifuge medium of varying density.
Other objects, features, and advantages of the invention will become evident in light of the following detailed description considered in conjunction with the referenced drawings of a preferred exemplary embodiment according to the present invention.